My boat sat untended (and unvisited) at the slip for at least 3 years, and possibly much longer. Some of the dock lines were rotted but still holding. The fenders were all hanging from the life lines via clove hitches. The lines were pretty much done, but not one had come undone on its own, and all of them came undone easily once the fenders were raised.
None of the lifelines show any damage or strain where the fenders hung, however, this is a very calm and sheltered lake, and the marina is even more sheltered and boats at the back end of the marina, back in a hole. If the boat was in an area where it was taking constant dock hits those fenders would have let go years ago, and there is no telling what damage they might have done to the lifelines.
Better would be to rig a separate line between stanchions and hang them off of that.